Persistent H. pylori infection is a risk factor for atrophic gastritis and distal gastric adenocarcinoma; however, only a small percentage of colonized persons develop neoplasia. Enhanced cancer risk may be related to differences in expression of specific bacterial products, to differences in host response to the bacteria, or to the specific interactions between host and microbe. H. pylori strains that possess the cag pathogenicity island induce more severe gastritis and are associated with an additional risk for developing atrophy and gastric cancer. A specific mechanism by which cagA+ strains may lower the threshold for carcinogenesis is by altering epithelial cell proliferation and apoptosis, processes that can be regulated by host inflammatory mediators such as prostaglandin products of cyclooxygenase-2 (COX-2). Over-expression of COX-2 in vitro inhibits apoptosis, and COX-2 is up-regulated within H. pylori-induced gastritis, atrophic gastritis, and gastric adenocarcinoma specimens. In vitro, H. pylori cagA+ strains stimulate COX-2 expression in gastric epithelial cells. Since we and others have shown that cagA+ strains are associated with increased gastric epithelial cell proliferation but attenuated apoptosis in vivo, induction of COX-2 by strain-specific microbial factors may represent a specific mechanism by which certain H. pylori strains heighten the risk for gastric adenocarcinoma. The long-term objective of this proposal is to examine the molecular mechanisms by which H. pylori strains selectively affect COX-2 regulated epithelial cellular turnover in vitro and in vivo. To address this, we will first determine whether H. pylori or secreted bacterial products alter COX-2-dependent apoptosis in a novel in vitro model of bacterial:gastric epithelial cell interaction (conditionally immortalized gastric epithelial cells). COX-2 expression will also be examined in myofibroblasts co-cultured with H. pylori and epithelial cells to more closely approximate events occurring within native gastric mucosa. Second, we will determine whether H. pylori infection affects COX-2-dependent cellular turnover in wild-type and COX-2 deficient mice. Third, we will investigate the role of specific H. pylori determinants on COX-2-regulated cellular responses by inactivating strain-specific genes identified by H. pylori whole genome microarray. H. pylori parental and isogenic mutant strains will then be co-incubated with conditionally immortalized cells and infected into mice. The effects of strain-specific bacterial factors and COX-2 generated products also will be investigated in a murine model of gastric carcinogenesis, INS-GAS hypergastrinemic mice. Systematic studies of each of these variables in vitro and in animal systems that reflect H. pylori pathogenesis in humans should help elucidate their relative importance, direct the course of future intervention and prevention strategies, and potentially provide a model of carcinogenesis arising within the context of chronic mucosal inflammation.